Field of Invention
The present invention relates to a technical field of support preparation, and more particularly to a method for preparing a support of a molecular sieve membrane.
Description of Related Arts
The molecular sieve membrane is a complete and compact aluminosilicate film having the cubic lattice; and the crystal skeleton thereof has many cavities of the same size, which enable the molecules having the diameter smaller than the cavity pore channel to pass through. Thus, the molecular sieve membrane is able to accurately adsorb or separate the molecules of the different size. The molecular sieve membrane is widely applied in the separation and purification of the organic solution and the gas, for example the separation and purification of the organic acids such as acetic acid and acrylic acid, the dehydration of ethyl alcohol and propyl alcohol, the purification of oxygen in air, and the carbon dioxide removal from the various hydrocarbon products. Compared with the conventional low-temperature distillation technology requiring a large amount of energy and equipment investment, the separation and the purification with the molecular sieve membrane can save a large amount of energy and equipment investment. Because the molecular sieve membrane depends on the cavities of the lattice skeleton of the crystal thereof to separate the small gas or liquid molecules, the crystal of the molecular sieve membrane should be continuous and compact, and the defects of the crystal film, such as holes and gaps, will cause the greatly decreased separation efficiency of the molecular sieve membrane.
Conventionally, the molecular sieve membrane is formed on the support through the in-situ hydrothermal synthesis method and the secondary growth method. The in-situ hydrothermal synthesis method is to attach the agglomerated, colloidal and unformed aluminosilicate material onto the carrier, and provide the appropriate supersaturation degree under the hydrothermal synthesis conditions, so that the crystal nucleus is formed at the carrier interface and grows into the compact film. The carrier serving as the support of the molecular sieve generally has an average pore diameter of about 1 μm, while the crystal nucleus formed through the in-situ growth method is generally nano-sized. It is unavoidable that the crystal nucleus is sunk into the pore channel, causing the collapse of the molecular sieve membrane and the generation of the holes. Thus, in order to overcome the above defect, the synthesis generally has a long time or repeats several times. The secondary growth method is to firstly synthesize the dispersed molecular sieve crystal particles through the hydrothermal method, then coat the molecular sieve crystals on the carrier, and arrange the carrier in the molecular sieve synthesis mother liquid for continuing the hydrothermal synthesis, so that the dispersed molecular sieve crystals continue growing and form the compact membrane.
Conventionally, the porous support, which has a median pore diameter of about 1 μm and is made of α-aluminum oxide, stainless steel and mullite, is generally adopted as the support of the molecular sieve membrane. The Chinese patent publication, CN102861516A, disclosed a method for producing a hollow fiber support which has a pore diameter of 0.9-1.3 μm and is made of α-aluminum oxide, wherein the hollow fiber support is for serving as the support of the molecular sieve membrane. The Chinese patent publication, CN104987120A, disclosed a single-pipe or multichannel tubular support which has an average pore diameter of 0.8-1.5 μm and is made of α-aluminum oxide and mullite. For both of the above two methods, the main material is the α-aluminum oxide, the average pore diameter is basically about 1 μm, the support is single-layered, and the pore diameter distribution is difficult to be uniform. No matter the in-situ synthesis method or the secondary growth method, the crystal nucleus or the crystal seed has the nano-sized particle size and thus is difficult to be uniformly coated on the carrier having the average pore diameter of 1 μm, so that the crystal nucleus or the crystal seed is unavoidably collapsed or embedded into the pore channel, causing the inconsistent thickness and even defects of the molecular sieve membrane. Meanwhile, for the support which is made of α-aluminum oxide or stainless steel, the material price is relatively high, the sintering temperature is relatively high or the sintering atmosphere is relatively severe, and the production cost is relatively high; if the molecular sieve membrane is prepared on the above support, the production cost is further increased. Because the molecular sieve membrane has a relatively short life, the high cost will limit the industrial application thereof. The cordierite serves as a low-cost ceramic material and has been reported to be applied in preparing the porous ceramics because of the good thermal shock resistance and corrosion resistance. However, because the sintering temperature and the melting decomposition temperature of the cordierite are nearly the same, the mixed sintering methods are always adopted, such as sintering the cordierite coarse particles with a great number of sintering aids, sintering the cordierite coarse particles with the cordierite fine powders, and sintering the cordierite coarse particles with the fine particle sintering aids prepared according to the cordierite composition, so as to sinter at a temperature lower than the decomposition temperature of the aggregate cordierite particles and prepare the cordierite porous material. For the above three methods, because the sintering aids or the fine particles inevitably block the pores of the cordierite coarse particles, the pore diameter of the porous material is certainly relatively large, otherwise it is difficult to prepare the cordierite porous material having the industrial prospect. Wang Yaoming et al. prepared the cordierite support, which is for high-temperature dust removal and has a pore diameter of 128 μm, through the cordierite powders of 300 μm with the sintering aids such as the potassium feldspar (Preparation of porous cordierite ceramic support materials for high-temperature dust gas, Journal of the Chinese Ceramic Society, 2011, Vol. 33, No. 10, pages 1262-1265). Han Huonian et al. obtained the porous ceramic support through mixing the coarse cordierite powders of 25 μm with the fine cordierite powders of 1.5 μm, and then sintering at 1400° C. The core of the above researches is to utilize the cordierite fine powders, the sintering aids or the fine ceramic powders prepared according to the cordierite composition, which have the lower sintering temperature, to bond the cordierite coarse particles having the higher sintering temperature, so as to avoid the decomposition and fusion of the cordierite coarse particles. However, the support consists of the coarse particle aggregate inevitably has a relatively large pore diameter, generally about 10-150 μm, and is unable to serve as a support of the molecular sieve support. Meanwhile, the fine powders or the sintering aids will block the pore channel, causing the decreased porosity. If directly using the cordierite fine powders, under the sintering temperature thereof, the fine powders are inevitable to be molten and decomposed, and the obtained ceramic is compact. Thus, it is difficult to prepare the cordierite porous support of the molecular sieve, having the pore diameter of 1 μm, through directly using the cordierite of 2-5 μm.